The present invention relates generally to charge coupled devices and, more particularly, to the adaptation of such devices for receiving and processing signals generated by photovoltaic infrared detectors.
When an infrared image is to be detected, it is customary to do so by arranging a plurality of infrared detectors of the photovoltaic type into an ordered array, which includes one or more columns of such detectors. The infrared image is scanned over the detector columns so that successive detectors in a given column detect a given portion of the image in sequence. In such an arrangement, the signals produced by detectors in a given column are applied to a signal shifting device so that the signals produced by all of the detectors in that column arrive at the output of the device at the ame time. In this way, random variations in the responses of various detectors are minimized by averaging their cumulated outputs.
A delay line with discrete components for carrying out this shifting and integrating function is disclosed in Laakmann U.S. Pat. No. 3,723,642 assigned to the present assignee. The desirability of replacing such a delay line with a CCD shift register is suggested in application Ser. No. 436,586, now U.S. Pat. No. 3,883,437 entitled MONOLITHIC IR DETECTOR ARRAYS WITH DIRECT INJECTION CHARGE-COUPLED DEVICE READ OUT, filed on even date herewith by D. J. Holscher, J. Nummedal, J. M. Hartman, and D. M. Erb and assigned to the present assignee. Also proposed in the referenced patent application is the desirability of injecting the signals produced by the IR detectors directly into the CCD without the use of an intermediate amplifier.
A recent development, charge coupled devices are by now well known, as exemplified for example by Kahng et al. U.S. Pat. No. 3,651,349 issued Mar. 21, 1972. In such a device when organized as a shift register, a dielectric layer is disposed over a surface of a semiconductive storage medium and a series of electrodes are disposed in or over this dielectric layer in a row so as to cause a series of depletion regions or "potential wells" to be formed in response to clock voltages which are applied to the electrodes periodically. The potential wells serve to store packets of electric charge which are injected into the storage medium through a P-N junction and the clocked electrodes in response to the voltages applied thereto transfer these charge packets along the potential wells.
When signals from a plurality of photovoltaic infrared detectors are to be handled by a CCD shift register so as to cause their sequentially produced outputs to arrive at the output of the shift register at the same time, the outputs of successively actuated detectors are applied to successive stages of the CCD shift register, each stage being usually represented by a respective pair of potential wells and the clocked electrodes which create them.
The direct application of the output of a photovoltaic infrared detector to a given stage of a CCD shift register tends to have some shortcomings in the absence of special provisions being made, such as the one which is the subject of the present invention.
The first problem created by direct coupling is that voltage spikes are coupled back into the injecting junction from the nearest clocked charge transfer electrode as a result of clocking pulses which are applied to that electrode. The reason for this coupling back into the injecting junction is that the nearest transfer electrode normally overlaps the injecting junction in order to have control over the charge which is to be transferred from the injecting junction to the potential well associated with that electrode. The overlap creates a coupling capacitance and it is through that capacitance that the voltage spikes are coupled back into the injecting junction. If a detector is connected directly to this injecting junction, the coupled back voltage spikes appear across the detector and affect its operation adversely.
Overlapping of the injecting junction by the first transfer electrode creates an additional problem for direct injection into a CCD. To wit, the detector can be utilized only during the ON period of the clocking pulse which is applied to the first transfer electrode. During this ON period, charges which are injected through the injecting junction are promptly swept away from that junction, thus enabling it to accept additional charges from the detector. During the OFF period of the clock, however, the detector cannot be used, because during that period, the incoming charges accumulate at the injecting junction and raise its bias voltage to an unacceptable level. This applies particularly to detectors which are bias sensitive, such as photovoltaic I. R. detectors. Such detectors have a limited tolerance for variations in the bias voltage to which they are subjected. Thus, such detectors could not operate properly if their bias voltage were permitted to rise to the levels which tend to result when charge buildup during the OFF clock periods is permitted. If the detector is then limited to operating only during the ON period of the clock pulse, its sensitivity will be reduced, since it is only operating during that fraction of the time which corresponds to the ON period of the clock pulse.
A signal might be injected from a detector into a CCD through a coupling capacitor. Such an approach has at least one shortcoming: the size of the capacitor necessary to accommodate low frequency bandpass characteristics associated with many detectors which would be used with CCDs would be excessive. The necessary capacitor size would be excessive for an additional reason: during the operation of the CCD, charge accumulates on the capacitor and, unless the capacitor had a very large value, the resulting voltage would raise the bias voltage of the detector to an unacceptably high level.
It is, therefore, a principal object of the present invention to effect direct injection of the signal from an I. R. detector into a CCD without adversely affecting the performance of the detector.
A related object of the invention is to provide direct coupling between an I. R. detector and a CCD without incurring bias voltage fluctuations, either through the accumulation of charge or through clock created transients.
It is a further object of the invention to provide an improved CCD which is adapted to receive signals directly from an I. R. detector while permitting the detector to generate a useful signal 100% of the time.
These and other objects of the invention are attained by providing a CCD of the type described hereinabove with means for individually injecting charge from one or a plurality of I. R. detectors into the storage medium near one or a corresponding plurality of its potential wells. An electrode structure is disposed between each of the injecting means and its associated potential well and a steady bias voltage is applied to the electrode structure so as to create thereunder a potential well. The electrode structure preferably includes a transfer portion nearest the injecting means and a storage portion nearest its associate potential well, with the potential well which is being maintained under the electrode structure, being deeper under the storage portion than it is under the transfer portion.
The transfer portion of the electrode structure functions as a transfer electrode between the injecting junction and the nearest clocked transfer electrode of the CCD. The coupling back of voltage spikes is prevented by the present invention since, there is no longer a need for an overlap between the injecting junction and the clocked transfer electrode nearest to it. Charge accumulation and bias voltage buildup are also prevented because the storage portion of the electrode structure of the present invention creates a potential well which acts as a reservoir into which charges can flow from the injecting junction even when the clock voltage on the nearest transfer electrode is OFF. This charge reservoir keeps the injection free of such charges and capable of accepting additional charges without raising its operating bias level.